The 2014 report found the Wollongong-Sydney route could be built by 2031.

702 ABC Sydney: Amanda Hoh

A new railway tunnel would shave 22 minutes off a train trip from Wollongong to Sydney, confidential documents reveal.

Key points:

It is estimated about 20,000 people commute from Wollongong to Sydney daily

The proposed rail tunnel from Thirroul to Waterfall would slash journey times to about an hour

Today, the journey takes more than 90 minutes

The ABC has seen a plan prepared for the New South Wales Government showing the commute would be cut from about an hour and a half, to 66 minutes.

The cabinet-in-confidence proposal estimates a tunnel built from Thirroul to Waterfall would cost $3.6 billion, a fraction of what it would cost to build the F6 motorway.

In last week's state budget, the Berejiklian Government allocated $15 million for planning of the F6 extension.

NSW Minister for Transport and Infrastructure Andrew Constance ruled out the rail investment.

"At this stage we have no plans to build a Thirroul tunnel," he said.

"I think it's fair to say the investments we are making are about providing more reliable and more timely services to the people of the Hunter and the Illawarra and the Blue Mountains but at this stage it would need to tie in with any long-term transport master planning which is what we're up to."

The Greens' transport spokesperson Mehreen Faruqi has questioned why the 2014 report Rail Corridors Strategy Sydney to Wollongong was never made public.

"This reduction of travel times by train will change the lives of tens of thousands of people living in the Illawarra and why isn't the Government pursuing it," she said.

'It would make my life a lot easier'

Jeremy Boulton travels from North Wollongong to the Conservatorium of Music in Sydney each day — a trip that takes him about two hours each way.

He said a shorter commute would make a big difference.

"It would mean I could get back and maybe take on some more work and a bit more money maybe to get me on the train everyday and it would really make my life just a lot easier," he said.

Labor's Illawarra spokesman and Keira MP Ryan Park said the faster journey time would deliver an economic boost for the South Coast.

The Government blueprint estimated the tunnel could be completed by 2031.

Mr Park said future transport plans for the region should prioritise rail.

"My priority has always been upgrades to the South Coast Rail line first, that's because its a public line, it's a line that does a lot of heavy lifting, we can move large numbers of people efficiently," he said.

Plans for a road option by extending the F6 freeway through southern Sydney to Waterfall have estimates ranging from $8 billion to $18 billion.

The road plan would controversially involve acquiring 60 hectares of the Royal National Park, or the demolition of homes and businesses on around 500 properties.

A Transport for NSW spokesman said the Government was always evaluating the state's transport options.

"We recognise the need for transport links between capital cities and other major cities for access to services and opportunities like business, specialist healthcare and international airports," he said.

"Over the next 20 to 40 years we need to ensure our cities and towns are better places to live, work and visit."